1. Field of the Invention
The present invention relates generally to aircraft flight performance computer systems and more particularly to a system for detecting and/or providing a warning of impending dangerous windshear conditions and for providing an avoidance command through the pitch director and throttle systems to enable the flight crew to avoid such conditions in a timely and safe manner.
2. Description of the Prior Art
Windshear is a weather condition which results in rapidly changing wind velocity and direction and in terms of aircraft flight performance is, of course, most dangerous during takeoffs and landing approaches. A number of attempts and proposals have been made to provide the flight crew of an aircraft with a warning of impending windshear conditions. One method that is presently in use is based on ground observations of wind conditions at sporadic locations around an airport. These wind determinations have been shown to be inadequate during several occurrences which have resulted in serious accidents. Other methods utilize on-board apparatus for detecting ground speed and comparing the ground speed to a measure of the airspeed of the aircraft and may include some sensing of vertical inertial acceleration. U.S. Pat. Nos. 4,012,713 and 4,079,905 are typical of such systems. A prior system previously described by one of the present inventors is based on the energy residing in the aircraft at any instant as it proceeds along a flight path, as in U.S. patent application Ser. No. 239,289, filed Mar. 2, 1981, now abandoned. A deficiency of the prior on-board systems is that while they may be effective in determining horizontal wind variations, they are inadequate for adequately determining vertical wind variations such as downdrafts and updrafts. Analyses of accidents that have been caused by severe wind variations near the ground indicate that they are composed of rapidly changing combinations of both wind components. These result in rapid changes in the magnitude and direction of the total acceleration vector which determines the aircraft's flight path in the vertical plane.
A further deficiency of prior art systems that utilize indicated airspeed (IAS) or calibrated airspeed (CAS) is that they do not take into account changes in the density of the air mass at varying temperature and altitude, thereby resulting in errors in the apparent windrate.
A typical windshear situation is graphically illustrated in FIG. 1. An aircraft approaching a runway 3 attempts to fly along a glide slope 4, defined as the straight line between P.sub.1 and P.sub.2. If a wind downburst exists along the glide slope, as illustrated in FIG. 1, the initial effect on the aircraft occurs at a point O.sub.1 on the glide slope, where it encounters an increasing headwind. This causes an increase in lift and a deceleration relative to the ground. The total acceleration vector, designated A.sub.VH, is the resultant of the inertial vertical acceleration component a.sub.V and the inertial horizontal acceleration component, a.sub.H. At point O.sub.1, the vector A.sub.VH points upward and backward with respect to the aircraft's path relative to the ground. Thus, there is a significant change in the magnitude of the resultant acceleration vector as the aircraft travels between P.sub.1, where there is no appreciable acceleration vector, and O.sub.1, where there now exists a sizeable acceleration vector.
The increase in lift at point O.sub.1 causes the flight path, 5 of the aircraft to go above the glide slope 4. There is also an increase in airspeed at O.sub.1 due to the inertia of the aircraft carrying it into the headwind and in spite of the fact that the aircraft is being decelerated relative to the ground. The pilot's normal reaction under these conditions is to reduce thrust and pitch the aircraft down to reduce airspeed and to realign the aircraft to its intended glide path 4.
As the aircraft passes the center of the windburst column, the headwind changes to a tailwind. The tailwind causes a decrease in airspeed, again due to the inertia of the aircraft carrying it into the tailwind and resulting in a forward acceleration relative to the ground. The decrease in airspeed now results in a loss of lift at point O.sub.2. The combined actions of the reduction of thrust at O.sub.1, the pitch down maneuver at 5, forward acceleration at O.sub.2 and the loss of lift at O.sub.2 results in a larger resultant acceleration vector at O.sub.2 which is directed forwardly and downwardly. If not corrected rapidly by the pilot by increasing thrust and pitching the aircraft nose up to increase its angle of attack, the aircraft will impact the ground violently at point 6. In the windshear example discussed above, it will be noted that during the transition from point O.sub.1 to point O.sub.2, the direction of the total acceleration vector A.sub.VH has undergone a large angular rotation on the order of 180.degree..
A windshear detector of the type sensing differences of an aircraft's inertial acceleration with respect to the sustaining airmass and capable of sensing and comparing the vertical and horizontal components of the aircraft's inertial acceleration with the vertical and horizontal components of the aircraft's acceleration relative to the airmass provides a first signal to a threshold detector responsive to signals above a predetermined magnitude. A second signal is computed from the magnitude of the detected windshear rate to provide an output representative of the maximum allowable duration the windshear can be tolerated for a given aircraft. The duration of the detected windshear rate signal is measured and compared with the computed time signal to generate a control signal from the differences of the aforesaid timing signals, so that an alarm or control system may be activated when the windshear persists for a time period greater than the computed allowable time. Thus, windshear rates of small magnitude persisting for a long time period will initiate an alarm, as will windshears of a high magnitude persisting for a short duration, so long as the detection threshold is exceeded.